been a SB guy all my life, but always dreamed of a BB SS...My new 1968 396/325HP is like brand new, done right,bored .040 over (404.3 ci)everything new, crank ground .010, roating assmbley balanced, all surfaces resurfaced, std valves,(hardened seats) 2.190-1.720... comp ratio is 9.47 to 1 for now the rear end is 10 bolt 3.08 tranny is M21 muncie (originalwith car was M20 with 3.73 rear end.)

I understand the redline of the 325 HP motor is 5000RPM...just how close to 5000RPM do I have to stay? LIke I said in my sbc, I used to shift at 7200 rpm, loved those screamers, butI know BB's are a different breed. I have a MSD 6AL unit with a 6000 RPM rev limiter in it, and I want to add a 100HP NOS bottle. Is this beautiful engine going to blow up if I miss a gear, or hit the NOS at the wrong time?
I value the motor too much to hurt it, but I would like some more G Force...maybe I should just change the rear end gears. I nee advise bad.

Take it to 6K, it will take it, I wouldnt be afraid to hit 6500 once in a while either. I used to run a 427 with a cast crank and a 125 shot,launch and shift at 6k with the button pushed.

The engine could take the RPM's, but can the cam and valve springs? If its a solid cam, it could likely take the 6500 hits once and a while that Rapid Robert mentioned. However that might be far out of the powerband. Some cam specs would help.

You say you want more kick in the pants. Keep in mind 396s aren't usually torque monsters. Especially if you got rectangular port heads on there. Even oval ports are pretty large for a ~400 ci engine. Then consider how much weight you've put on the car with a big block. I say get steeper gears, u'll get through the RPMs much quicker in each gear, and I think u'll have a lot more fun. At least untill you have to fill up sooner than before

thanks very much for the input, the cam is stock, actually the whole motor is a very nice stock rebuild. I had planned on just doing some bolt on stuff (already has headers, MSD, and I plan to add NOS)because the motor is so new, nice and fresh.

I feel better knowing that 6 grand every once in a while won't hurt it.

I think I should look for a 12 bolt (has 10 bolt 3.03 now) with some lower gears, BUT, I want to be able to drive the thing on the hiway. What gears maker a better compromise? 3.73? Is 4.10 too much for road trips?

I had a Camaro sb with 4.88 and I don't ever want to take another road trip in a car like that...tho she did mighty fine at the strip smile.gif

thanks again

I would go for 3.55-3.73 gears if you keep the M21
With a stock 325 HP cam,revving it to 6K is a waste.You could install a mild cam that will make power up to 6k,or just leave it alone and powershift it at 5000 rpms..and it might rev to 5500 in between the gears,which would be fine.
My 454 has a mild cam,and the best shift point for it is about 5500-5700,but it revs freely under load to 6K+

Exactly what Mr. 4 speed said. If I remember correctly the 325/396 had a hyd cam with a .393 lift and a duration of about 204-206 @ .050. Not much left there after 5200-5500 rpm. The engine will easily take it but you will start to go into valve float because of the weaker factory springs.

I had an Isky 280 hyd Mega cam in my 427 and it went to 6500-6700 shifting at 6000. Even the Isky 270 would be a better choice if your just wanting a liitle more oomph. (4.10 gears with M20, 3700rpm @ 65mph on the hwy sucked)

For reference. I have a friend who races in stock eliminator with a 396/325. Rules require a factory lift cam with whatever duration you want. With a whole hell of alot of duration, they still only run the motor to 6500, shifting at 6100-6200 and they are a full second under the index. Lots of valve spring pressures in there too.

I think with those oval ports you could run the 3.55's and that 10bolt and still cruise very comfortably. (with a slightly larger cam and matched springs and 1.88 exh valves)

Just my .02. Good Luck
Rob

From what I remember from that motor, I agree that it didnt seem to be worth trying to spin anything over 5500really. Also, I think a 100 shot will be fine as well. But I would add in some safety stiff with the kit, especially with the 4 speed. Youve already got yourself a rev limiter, and you should surely add a wide open throttle switch as well. That way, when you let off the gas between gearshifts with the 4 speed, your not pumping nitrous in there, cause the nitrous system will cut out until you get back on the pedal. And thats good cause for the sake of your engine, you dont want to be gassing the motor when you arent running at WOT.

that motor is build with the same stroke as the high winding 427 if you have correct valve springs a nice solid cam 7 grand if its hyd 6500 / i had a rex hucthinson build 468 gm sreel crank manley rods 12.5.1 comp and spun it hard into 8k many times its all in how its built imo but the 396 6-6500 easy . now if your gonna drive it that way everyday then hey anythying could happen

ya I'm going to have to agree with Mr 4-speed, I was under the impression that with a M20 you need a rear end gear of 3.55 or longer (numerically lower) and for the M21 and M22 you need a rear end gear of 3.73 or numerically higher, I know that when I first put in my M21 and still had the stock rear end gears (2.74ish) I couldn't get off the line because the first gear in the M21 is a 2.20, it was soo bad that I couldn't drive it because when I did it looked like I didn't know how to drive a stick shift because of the stumble off the line. Gear multiplication is really what makes the "G's" and they will make or break the car. I would suggest replacing the rear end gears first to a 3.55 if you still want some freeway speed. that will make your car a ton faster. but this isn't what you asked...well I can't help you with the BB thing I have one of those little motors. <p>just my 2 cents

That cam is 198 duration at .050" and only .398" lift!!

Yes, a CC 270 Magnum at 270/224, 110/106, .510" lift, or

Isky 270 Mega at 270/221, 108/108, .542" lift along with needed springs will really liven her up!!

pdq67

thanks so much for all the imput guys, it's nice to look at all the options you suggested...one more thing, what was the difference between the 396 motors, 325hp,360 hp, 375hp,? was it just some compression and some cam?

Did they all have 4 bolt mains?

Route66, The weakest parts of your combination are the pistons and valvesprings. The cam in the 325 H.P. 396 just isn't any good for anything but hiway driving and gas mileage. The 350 H.P. cam on the other hand is pretty good and still gets very good mileage or one of the previously mentioned aftermarket cams. The cast pistons and wristpins aren't very strong but if someone were to replace those the rest of the engine is very strong. Anybody wanting to pump up the performance of a big block should re-cam it and put in forged pistons then your ready to rock and roll. Allways buy the whole cam and kit so as to protect yourself from valve dropping woes. The 375-396 is completely different internally from its little brothers on the inside and is the only one with four bolt mains. Also has forged rods, forged pistons with 11-1 compression, solid lifter cam, rectangular port heads, and an aluminum intake with a 780 holley carburetor.

I have a similiar motor to you. Mine started life as a '70 402 motor. now .030 over to 408CI. Motor had a cast crank in it [cut .010/.010] and forged pistons when I got it and still has the same crank and pistons. Don't quote me on this, but a lot if not most 396 cranks were n Nodular iron, not cast iron. Big difference in strength. You can check your's out, part number should start with "N". Nodular cranks are tough. Yeild and tensile as high as steel, but nodular iron pours with much less porosity. I knew plenty of guys that ran the cast nodular cranks over the years, they make for alighter rotating assembly. As long as the rods are done correct and the bearing clearances are right, go ahead and run it. I have had mine to 6200 - 6300 many times, it actually has a sweet sound at RPM.

Gearing wise, a 3.73 with a 275-60/15 (28" tall) works well. If you run a shorter tire, maybe drop to a 3.55.

Your cam will be done by 4800 RPM but with the rest of the drivetrain it is about the only one that will work. If you want to keep the tall gears think about a ST-10 with either a 2.88 or 3.42 low.

had a 69SS 325hp back in 74. i put 4.10 gears in it and had h60-14 tires on it,255/60-14 would be about the size in todays lingo. at the strip i blew away first and second gear and by the time i hit forth and wound it out i had to push in the clutch and coast the last 100 feet or so of the 1/4 mile, it had maxed that little 325 hp cam out and i started to hear valve float. turned a 14.3 if i remember right. so no i don't recomend 4.10 with a 325 hp cam.